A vehicle may include a power plant for providing power to operate a vehicle and a transmission for converting at least a portion of the power to work in order to propel the vehicle. The speed of the vehicle may be controlled by controlling the output of the vehicle's power plant and/or the by selecting a gear ratio of the vehicle's transmission. For example, a vehicle may include an internal combustion engine configured to operate within a range of engine speeds and a transmission configured to convert the engine speed to an output speed such that the vehicle travels at a speed that varies according to a combination of the engine speed and the ratio of the engine speed to the transmission output speed.
Some vehicles may include, for example, an internal combustion engine and a powershift transmission. For such vehicles, the output of the internal combustion engine may be operably connected to the powershift transmission via, for example, a torque converter configured to transfer power from the internal combustion engine to the input of the powershift transmission. The powershift transmission may include a gear assembly along with one or more clutches for selectively engaging combinations of the gears such that the ratio of the transmission's input speed to output speed may be selected to provide discrete gear ratios.
For some vehicles, such as work machines, it may be desirable to operate the vehicle within predetermined speed ranges. Each of the predetermined speed ranges may generally correlate, for example, to a particular gear ratio engaged in the transmission and a range of engine speeds of the internal combustion engine. For example, the transmission may include gears and clutches that permit the transmission to be engaged in three forward gear ratios and three reverse gear ratios. A vehicle with such a transmission configuration is able to operate in six speed ranges—three forward speed ranges and three reverse speed ranges, each corresponding to an engine speed and one of the six gear ratios. In other words, a vehicle having such a transmission configuration is limited to six speed ranges by virtue of the number of available speed ranges being limited to the number a transmission gear ratios.
One possible drawback of such conventional systems may relate to their lack of ability to provide a vehicle operator with more available speed ranges than the number of gear ratios available in the transmission. Providing more speed ranges than available gear ratios may be desirable, for example, to more closely tailor operation of the vehicle to an operator's preferences. For example, an operator may select a speed range based on considerations, such as, for example, the type of operation being performed by the vehicle (e.g., grading on relatively flat terrain or grading on relatively steep and/or uneven terrain), the condition of the terrain on which the vehicle is traveling, and/or the level of skill of the operator. By providing more available speed ranges than transmission gear ratios, an operator may be able to more closely tailor operations of the vehicle to the operating conditions and/or the operator's skill level. Another possible drawback with conventional systems may relate their inability to control the vehicle's speed as the engine experiences changing loads. It may be desirable to control operation of the engine and/or transmission such that the vehicle substantially maintains a desired speed in a manner substantially independent of the magnitude of the load experienced by the vehicle's powertrain.
An example of a transmission control system for a vehicle having an internal combustion engine and a powershift transmission is described in U.S. Pat. No. 5,526,261 (the '261 patent) issued to Kallis et al. on Jun. 11, 1996. The '261 patent describes a transmission control system including a first electronic controller for providing transmission control signals required to command the powershift transmission to shift and operate in any particular one of its forward gears, and a second electronic controller for providing at least one engine control signal required to command the engine to operate at one of a number of different desired engine speeds and to monitor the actual engine speed. In the '261 patent's system, gear ranges are used that slightly overlap along with a throttle that is infinitely adjustable within a range of engine speeds, such that any given ground speed within the range of possible vehicle speeds can be achieved and maintained, so long as the engine is operating within its overall power envelope.
Although the system described in the '261 patent may control the vehicle's engine speed and powershift transmission's forward gears, the '261 patent's system still suffers from the drawbacks outlined above, such as, for example, an inability to maintain the vehicle's speed as the powertrain experiences changing loads.
The disclosed systems and methods for controlling vehicle speed are directed to overcoming one or more of the drawbacks set forth above.